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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - WebKit incorrectly clips position:fixed element, inside of its "position:relative; overflow: hidden" parent, IF that parent has "z-index" set"
href="https://bugs.webkit.org/show_bug.cgi?id=160953">160953</a>
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<th>Summary</th>
<td>WebKit incorrectly clips position:fixed element, inside of its "position:relative; overflow: hidden" parent, IF that parent has "z-index" set
</td>
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<th>Classification</th>
<td>Unclassified
</td>
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<th>Product</th>
<td>WebKit
</td>
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<th>Version</th>
<td>Safari 9
</td>
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<th>Hardware</th>
<td>Unspecified
</td>
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<th>OS</th>
<td>Unspecified
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>Normal
</td>
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<th>Priority</th>
<td>P2
</td>
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<th>Component</th>
<td>Layout and Rendering
</td>
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<th>Assignee</th>
<td>webkit-unassigned@lists.webkit.org
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<th>Reporter</th>
<td>dholbert@mozilla.com
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<th>CC</th>
<td>simon.fraser@apple.com
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<pre>STR:
1. Load this testcase: <a href="https://jsfiddle.net/bo4q2hbh/">https://jsfiddle.net/bo4q2hbh/</a>
EXPECTED RESULTS:
Teal div should not be clipped at orange border. (The teal div is a fixed-position element, and it should use the viewport as its containing block, rather than using its orange-bordered relpos ancestor.)
ACTUAL RESULTS:
Teal div is clipped at orange border! (in Safari)
I get EXPECTED RESULTS in Firefox 48, Edge 14, and Chrome 54.
I get ACTUAL RESULTS in Safari 9.1 (on Mac OS El Capitan)
The bug only happens if the relatively-positioned ancestor has "z-index" set. In my above-linked jsfiddle, that element has "z-index: 0". If I remove that line, the bug goes away and I end up with EXPECTED RESULTS.
Note: Chrome has a similar bug, but their bug is more restricted (it only happens if there's also 3d-transformed descendant).
<a href="https://bugs.chromium.org/p/chromium/issues/detail?id=638780">https://bugs.chromium.org/p/chromium/issues/detail?id=638780</a></pre>
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